Ballistic resistant articles which contain high strength fibers for use as structural members and panels, like spall suppression liners, for a variety of military applications are known. Thus far, one of the most successful fibers is a polyaramide, more specifically poly(phenylenediamine terephthalamide). Such filters in fabric form are in their usual method of use encapsulated or embedded in a composite article of a thermoset resin. Military Specification MIL L 62474B provides a one such laminate or composite employing a polyaramide and a laminating resin which is the cured reaction product of polyvinyl butyral, phenol formaldehyde, and trimethylol phenol and phthalic anhydride. Such laminates have utility for armor applications and spall liners, but one of their more notable deficiencies is that they are not easily machined.
In order for a composite to have its broadest application for a wide variety of ballistic resistant applications, it is important that it possess a wide balance of important properties. The properties include, of course, proper ballistic performance; that is the composite functions satisfactorily as a ballistic system be it structural armor or a liner. Other required properties are that the material be stiff and self-supporting, fire-resistant, smoke-resistant, non-toxic, machinable and that it be capable of being cut, trimmed and drilled with, if need be, carbide or diamond tips. Since one of the uses may be structural armor it is important that the materials and fabrication process be capable of producing thick composites.
A composite having these properties has been identified and is formed from woven glass roving made from high strength magnesia aluminosilicate glass fibers and an unsaturated isophthalic-maleic-propylene glycol polyester resin which can be cured to a thermoset condition with styrene and a free radical peroxide initiator. A resin solution (E-701 available from Owens-Corning Fiberglas Corporation) which has been identified as producing quite satisfactory results is a system of a low acid number polyester namely one having an acid number less than about 12.5. Unfortunately, ballistic resistant composites of this type have only been made by a manual hand layup technique because of the properties of that resin system. This technique is not well adapted for the reliable production of such composites in an expedient commercial and economical manner.
In accordance with the present invention as economical and commercially satisfactory process is provided for manufacturing such composites especially composites of substantial thickness. Typically the composites can be formed having thicknesses on the order of at least about 1/8 inch and the process is surprisingly capable of producing high quality composites having thicknesses of 1/2 inch, 3/4 inch, and higher, up to and including 21/2 or even 3 inches in thickness. Additionally, the process can be practiced so as to form rectilinear or curvilinear materials. Thus any of the wide variety of shapes needed for a wide variety of military applications including, for example, tank turrets can be expediently manufactured by the present process. Those skilled in the art will readily appreciate the significance, and unexpectedness of being able to form thick rectilinear and curvilinear ballistic resistant materials. Considering the fact that the resin is cured by the use of monomeric styrene and a free radical initiator, one would not expect that such thick composites could be expediently formed because of problems with their cracking internally as a result of the exothermic reaction.